refactor(engine): expose the layout validation contract to engines

Engines had to reverse-engineer the benchmark validator: Opus55 assumed a
0.01 arc tolerance (the validator uses 0.001), Gpt6Astra added hand-tuned
paddings and copied the validator's check order, Qwen picked its chord
tolerance to stay under a constant it could not reference.

NestTolerances publishes the validator's arc tolerance, the Clipper grid
and SafeClearanceMargin (with its derivation). NestLayoutCheck moves the
benchmark NestValidator's checks into OpenNest.Engine as a public API
(Clears for a part pair, Violations for a whole result); NestValidator is
now a thin wrapper. Verdicts are unchanged: tests compare ordered
violation lists against a frozen copy of the old validator, and a
tangent-disc stress test covers 432 pairs at the safe margin.

Co-Authored-By: Codex <noreply@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Engine/Jobs/NestJobPlacementValidator.cs

@@ -12,7 +12,7 @@ internal static class NestJobPlacementValidator
     // Flattening for placement overlap/spacing checks: the same 0.001 the benchmark's
     // NestValidator and Part.Intersects use. Arcs are inscribed, so a layout placed exactly at
     // the spacing passes; outward arcs may come up to this much closer than the spacing.
-    private const double PlacementChordTolerance = 0.001;
+    private const double PlacementChordTolerance = NestTolerances.ValidationOutline;
 
     internal static void ValidateCandidate(
         PlateCandidate candidate,